The time evolution of a phase-separated morphology in a binary polymer mixture involving a percolation-to-cluster transition (PCT) was investigated by a combined use of laser scanning confocal microscopy (LSCM) and light scattering (LS). We constructed three-dimensional images of phase-separating structures in the PCT process by means of the LSCM technique. The three-dimensional image observed by the LSCM was found to be consistent with the result from the LS, revealing that the phase separation process can be classified into four regimes: (i) percolation regime, (ii) PCT-I regime, (iii) PCT-II regime, and (iv) cluster regime. Details of phase separation dynamics in each regime are described in the text.